High-power control means for attenuating microwave energy

ABSTRACT

Compact high-power control means for attenuating electromagnetic energy at microwave frequencies are disclosed providing energy reflection as well as adjustable absorption load means, in a terminated branch line coupled by energy splitting means to a first transmission line. Adjustment of the load-to-line coupling results in variation of the magnitude of the reflection and control of the energy propagated in a transmission system with negligible insertion loss and substantially flat attenuation frequency response. The device exhibits no frequency-sensitive characteristics to thereby afford a broadband frequency response.

United States Patent Henry W. Perreault Chelmsiord, Mass.

31,707 Apr. 24, 1970 Nov. 30, 1971 Raytheon Company Lexington, Mass.

[72] inventor [2 i Appl. No. [22] Filed [45] Patented [7 3 Assignee [54] HIGH-POWER CONTROL MEANS FOR ATTENUATING MICROWAVE ENERGY 13 Claims, 6 Drawing Figs. v

[52] US. Cl 333/81 8, 333/10, 333/22 F [51] lnt.Cl 1101p 1/22 [50] Field oi Search 333/10,8l, 81A,81B,22,22F

[56] References Cited UNITED STATES PATENTS 2,920,292 l/l960 Scovil et al. 333/10 X OUTPUT 2,755,383 7/1956 Mannheimer 333/10 X 3,182,203 5/1965 Miller 333/l.1 UX 2,579,327 12/1951 Lund.... 333/81 B X 2.605.400 7/1952 McClain,Jr. 333/81 B UX 2,853,687 9/1958 Weber 333/81 B Primary Examiner- Herman Karl Saalbach Assistant Examiner-Paul L. Gensler Auorneys-Harold A. Murphy, Joseph D. Pannone and Edgar 0. Rost ABSTRACT: Compact high-power control means for attenuating electromagnetic energy at microwave frequencies are disclosed providing energy reflectio'n as well as adjustable absorption load means, in a terminated branch line coupled by energy splitting means to a first transmission line. Adjustment of the load-to-line coupling results in variation of the magnitude of the reflection and control of the energy propagated in a transmissionsystem with negligible insertion loss and substantially flat attenuation frequency response. The device exhibits no frequency-sensitive characteristics to thereby afford a broadband frequency response.

PATENTEUunvsmsn 35245 5 sum 1 or 3 OUTPUT .ZNI/ENTOE HENRY W. PERREAULT ATTORNEY j-ao .Z'NI/ENTOE HENRY W. PERREAULT ATTORNEY M w m PATENTED NUV30 1971 SHEET 2 or 3 FIG. 3

PATENIEU rmvso |9n SHEET 3 [1F 3 I I l2 l6 TURNS C.C.W.

O O O O 6 4 2 INVENTOI? HENRY W. PERREAULT ATTORNEY HIGH-POWER CONTROL MEANS FOR A'ITENUATING MICROWAVE ENERGY This invention herein described was made in the course of a contract and subcontract thereunder with the Department of the Navy.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to microwave energy attenuating and control devices for high power electromagnetic wave transmission systems.

2. Description of the Prior Art In transmission systems for the propagation of electromagnetic energy, particularly at microwave frequencies, the continual demand for higher power levels presents a continuing problem in the attenuation and control of the transmission of such energy utilizing known microwave components which are generally adaptable and suitable for only handling low power levels of less than 25 watts. Rather cumbersome attenuator and power divider systems for handling higher power levels have evolved in the art to meet these needs. High-power systems commonly include mode transducers, as well as numerous other components including rotating midsections, together with extremely long external energy-absorbing loads which are relatively limited by the peak power capability. The variable attenuators for such systems are quite elaborate with numerous sidewall couplers and phase shifters commonly consuming many square feet in area. The cost and weight problems of such high-power handling systems, therefore, have been a limiting factor.

Electrical performance characteristics, however, have been even more discouraging since the addition of the large number of prior art components has only resulted in intolerably high insertion loss values over the frequency bands of interest. Additionally, all prior art attenuation and control devices heretofore developed for high-power systems are extremely frequency sensitive and hence, unsuitable for broadband frequency applications. Improved devices, therefore, of reduced overall mechanical configuration and greatly reduced insertion loss characteristics over relatively broad frequency bands are essential for more effective utilization of high-power microwave energy systems.

SUMMARY OF THE INVENTION In accordance with the teachings of the present invention a broadband high power control device is provided with compact variable attenuator means. In a copending patent application Ser. No. 846,397, filed July 3], 1969 by Henry W. Perreault, now US. Pat. No. 3,560,888 issued Feb. 2, 1971, a unique microwave energy termination device is disclosed having exceedingly high-power absorption capabilities. The energy-absorbing means are positioned within a transmission path oriented perpendicularly to the main transmission path. The end of the energy-absorbing means and the entrance to the perpendicular path are spaced a frequency dependent distance from an energy reflecting end wall terminating the branch transmission line. A lossy dielectric medium of a ceramic or plastic composition, as well as dielectric fluids, are utilized as the absorbing means. By various impedance matching means the termination device will provide low-voltage standing wave ratio characteristics over a relatively broad frequency band.

In order to provide in the present invention for an adjustable power control device having a substantially flat attenuation response over a broad frequency range, adjustable energy-absorbing load means are mounted in a waveguide branch line terminated by an energy reflecting member. This line is coupled by conjugate energy splitting means such as a 3 db. hybrid junction and directional coupler means to the sidewall of a first high-power transmission line. Movement of the energy-absorbing means to control the depth of insertion of the energy-absorbing means within the terminated line results in variation of the reflection coefficient characteristics, as well as phase tracking of the high-power energy propagated in a first transmission line. The shunting absorbing means in the terminated branch line appear as shunt conductances over a wide frequency band. As a result the disclosed control device provides a unique feature in that it does not exhibit any frequency sensitivity similar to all prior art attenuating and control devices.

Exemplary embodiments of the invention to be described for all frequency bands will provide peak power handling capabilities up to 10 megawatts at L-band and average power capabilities of 10-25 kilowatts. At X-band such embodiments are capable of handling a half a megawatt peak power and SOD-watts average power in a structure having an overall height of only 4 inches and a branch line overall length of a similar value of 4 inches. The measured value for insertion loss is negligible at 0. l5 db. The input voltage standing wave ratio maintained over the band in each instance was well below l.l5 and phase shift was also maintained at a low value. Further, in an embodiment at S-band the amount of power transmitted is adjusted from 10 to percent of its original value by only 32 turns of the actuator arrangement.

The new control device, therefore, provides a means for filling the abrupt gap between low power and high-power attenuating devices available in the art. The many applications include its use as a high-power attenuator with test sources to provide greater flexibility in laboratory applications and extend the range and scope of all component testing facilities in the microwave art. In addition, the disclosed invention is ideally suited to drive controls for very high-power oscillator and/or amplifier tube chain systems, as well as remote control power level tracking, by reason of the absence of any moving attenuation components which lead to poor reliability and arcing or sputtering problems. Other applications requiring highpower' energy control devices include linear accelerator, as well as food and industrial processing systems.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, as well as the details for the provision of preferred embodiments, will be readily understood after consideration of the following detailed description and reference to the accompanying drawings, wherein:

FIG. I is a top view of the illustrative embodiment with a portion of the hybrid junction coupler removed to reveal internal structure;

FIG. 2 is a side elevation view of the embodiment of the invention shown in FIG. 1;

FIG. 3 is a detailed vertical cross-sectional view taken along the line 3-3 in FIG. 1;

FIG. 4 is a plot of a tuning curve of an exemplary S-band device embodying the present invention in relation to the percentage of power transmitted through the transmission system;

FIG. 5 is a cross-sectional view of an alternative absorbing load means for utilization in the embodiment of the invention; and

FIG. 6 is a front elevation view partly in section of the probe coupled absorbing load means of the type disclosed in FIG. 5 utilized in the embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIGS. 1 and 2 the transmission system 2 comprises a first input waveguide section 4 and a second out put waveguide section 6 with a partition member 8 disposed therebetween. Conventional waveguide mounting flanges 10 are attached adjacent the ends of the waveguide sections 4 and 6. Coupled to a sidewall of each waveguide section 4 and 6 is a 3 db. hybrid junction coupler 12 defining juxtapositioned passageways 14 and 16 with a common wall member 18 having a coupling iris 20. The hybrid junction coupler is a well-known energy splitting means in the microwave art and provides for an efficient four branch network to conjugate electrically the energy therein with the reflected energy returned by output port 22 to the second line. The opposing ends of passageways l4 and 16 are connected to juxtapositioned waveguide sections 24 and 26 having a common wall member 28 therebetween. The waveguide sections are terminated by an energy reflecting conductive end wall 30. Each waveguide section 24 and 26 supports an energy-absorbing load means shown generally as 32 and 34 adapted to extend therein. Simultaneous actuation by common control means 36 regulates the degree of insertion and coupling of energy between the absorbing means and terminated line. Hybrid junction coupler 12 together with the waveguide sections housing the adjustable energy-absorbing means and terminating in end wall member 30 collectively comprise the terminated line 38 coupled to the transmission system 2. Preferably, the dual energy absorption load means are coupled at a point one-quarter of a guided wavelength from the end wall 30 as taught in the aforereferenced copending patent application.

The incidence of microwave energy on first line section 4 results in the introduction of this energy at the transmitted high power level to the input port 21 of the 3 db. hybrid coupler 12. In the hybrid junction the energy is divided into two portions with one-half traveling through passageway 16 and the remaining half traversing the coupling iris to enter the passageway 14. In accordance with the well-known functioning of a hybrid junction coupler the energy in passageway I4 undergoes a phase shift to arrive at waveguide section 24 lagging by 90 the energy traversing the adjacent passageway 16. The energy reflection, as well as absorption means, provided within waveguide section 24 and 26 provide any desired degree of attenuation and the resultant reflection coefficient is determined by the depth of the insertion of the energy-absorbing means within waveguide sections 24 and 26, to vary the degree of coupling. Energy entering the respective waveguide sections 24 and 26 is suitably attenuated to achieve the desired power transmission factor and the reflected energy reenters hybrid junction coupler 12 to traverse this component in the opposite direction.

Each time energy traverses the coupling iris 20 it divides in half with one-half propagating directly through to the aligned port at the opposite end of the hybrid junction coupler and the other half crossing over to the other passageway. In view of the 90 phase lag with each traversal of the coupling iris 20 the half of the energy reflected through passageway 14 arrives at input port 21 180 out-of-phase with the energy arriving there from passageway 16. As a result of this phase difference a cancellation takes place and no energy will be redirected through input port 21 to the first waveguide section 4. The remaining energy from passageways l6 and 14, however, arrives at output port 22 in phase and the resultant sum of the microwave energy is propagated to the second line through the output waveguide section 6 at the new power level relative to the original incident high-power energy received at the first input waveguide section 4.

Referring next to FIG. 3 the energy-absorbing means 32 and 34 will now be described. Since each energy-absorbing means is essentially similar, numerical designations will be shown on only one of such members for the sake of clarity. A cylindrical housing member 40 is secured at one end to waveguide section 24 and collar member 42 by metallurgical sealing techniques. A fluid coolant-containing shell member 44 of a dielectric material is provided with tapered section 46 to provide impedance matching means of the energy-absorbing means to the waveguide section 24. A material such as quartz glass or plastic materials such as those available under the trade names Vicor" or Rexolite are preferably employed for member 44. The length and slope of the tapered section 46 will follow well-known microwave art techniques. The upper end of shell member 44 is joined between end plate member 48 and threaded sleeve 50 by means of threaded end cap 52 which engages the outer threaded portion of sleeve 50. Plural O-ring members 54 and 56 maintain a liquidtight relationship by sandwiching the flanged lip 58 of the shell member 44 between the end plate 48 and sleeve member 50. The end plate member 48 also supports inlet and outlet fluid conduit means 60 and 62, respectively to provide for the flow of a fluid dielectric coolant for absorption of the microwave energy. Sleeve member 50 is provided at its opposing end 64 with a notched portion 66 and O-ring member 68 in contiguous relationship with the outer wall of the cylindrical housing member 40. A plate member 70 is permanently secured to the dual cylindrical housing members 40 and provides an anchor surface for end 72 of rotatable manual actuator 74. Intermediate to the anchor plate member 70 and end cap member 52 is a ganging plate member 76 secured to each sleeve member 50 and threadably engaging rotating actuator 74. Rotation of the actuator 74 results in movement of the ganging plate member 76 coupled through universal joint 78 to simultaneously raise and lower the sleeve members 50 and thereby control the depth of insertion of shell members 44 within the waveguide sections 24 and 26. A greater degree of insertion will result in considerably more absorption of microwave energy and thereby less reflection from terminal end wall 30 to be returned through the hybrid junction 12. By careful calibration of the varying degrees of insertion of the energy-absorbing means 44 accurate control of the degree of desired power transmission may be achieved.

The foregoing embodiment provides what is referred to in the art as dielectric coupling by reason of the exposure of the dielectric materialto the microwave energy. It may be noted that inlet conduit 60 while shown disposed in a diametric arrangement may be centrally positioned with respect to the outlet conduit 62. This component may also be lengthened to provide for the flow of the fluid coolant closer to the apex of the tapered section 46. The additional exposure of additional conduit material adjacent to the apex of the tapered section 46 will also provide additional impedance matching means together with the tapered section. The manual actuator is conventionally driven by means of a knob and handle arrangement (not shown) or any other suitable driving means. In the exemplary embodiments of the disclosed invention the shell members 40 were fabricated of brass tubing. At this juncture it may also be noted that reflecting end wall 30 may be provided with adjustable conductive tuning means to provide for further matching of the impedances of the absorbing load to the line, as detailed in the aforereferenced copending patent application.

The value of the invention may be readily appreciated by reference to FIG. 4 illustrating a plot of a tuning curve of a device having a circulating fluid coolant absorbing load of the type described in reference to FIG. 3 covering a frequency range of between 2.8 and 3.2 GHz. and providing a maximum voltage standing wave ratio of 1.10 or less. The peak power handling capability was 7 megawatts and the average power rating was 10 kilowatts. The flow of the fluid coolant through the dielectric shell members was approximately L75 gallons per minute. This device was calibrated in relation to the number of turns required of the rotatable actuator 74 to control the depth of insertion of the dielectric members 44. Curve 80 represents the tuning rate relative to percentage of power transmitted at the low end of the band while curve 82 provides a representative tuning curve at the high end of the band. It will be noted that 32 turns counterclockwise will encompass a range of 10 to percent of the power transmitted. It is possible, therefore, for the person employing the disclosed invention to carefully control the power transmitted through a system by the number of turns relative to the percent of power required. The exemplary embodiment exhibited a very flat attenuation response over the frequency band by the ganged positioning of the energy-absorbing loads within the waveguide sections. The overall height of the energy absorbing means in this embodiment was only 13 /8 inches and the complete power control device extended only 17% inches from the main line.

FIG. 5 is illustrative of an alternative embodiment for the energy absorbing means and is of the so-called reentrantcoaxial-type water load." Generally, such water loads circulate the fluid coolant in a more efficient manner and provide a plurality of flow paths within the fluid retaining member. An example of such devices is disclosed in copending patent application, Ser. No. 889,393, filed Dec. 31, 1969 by Henry W. Perreault. Such loads are inserted in the same manner as the load disclosed in FIG. 3 and will be generally referred to by the numeral 84. The shell member 86 is again fabricated of a suitable energy permeable material such as quartz glass or plastic materials. Coupling of the energy from the terminated line to the energy-absorbing load is provided by means of a conductive member 88 which in the present embodiment will act as a probe to provide for coupling to the line in place of the dielectric coupling heretofore described. Again the shell member 86 may be tapered adjacent its inner end and in the disclosed embodiment an elongated taper has been provided. A center fluid conductor 90 threadably engages probe member 88 adjacent its inner end and provides with suitable seals a fluidtight joint. An outlet conduit 92 is also provided and supported within end member 94. The fluid entering conduit 90 is distributed by means of radially disposed opening 96 near the apex of the shell member 86 and then may be distributed through any number of paths in either a series or parallel flow as taught by the aforereferenced copending patent application. It is understood, of course, that any coaxial water load of this type as well as the one disclosed in U.S. Pat. No. 3,044,027, issued July 10, 1962, to D. D. Chin et al. and entitled "Radio Frequency Load may be employed. The central conductor 90 is tapered adjacent its inner end as at 98 to also provide impedance matching means for the energy absorbing means.

A pair of the disclosed energy absorbing loads in FIG. 5 are mounted in the same manner as the previously disclosed energy-absorbing loads and reference is now directed to FIG. 6. The hybrid junction coupler, as well as central portion of the first input and second output line waveguide sections 4 and 6, have been broken away to reveal the internal structure disposed within waveguide sections 24 and 26. Housing members 40, as well as the accompanying mechanical actuating structure, are identical to that shown in FIG. 3. The coaxial water loads 84 extend within housing members 40 with the probe members 88 exposed to the incident microwave energy which is coupled to the energy-absorbing load means. It may be noted that in this embodiment the probe members 88 have been slightly rounded at their ends to avoid any mismatch conditions or arcing. The overall energy-absorbing means are adjusted in the manner described herein and the depth of the probe, then, provides different values of attenuation and reflection. Complete attenuation of the power transmitted through the system would naturally occur at the full depth of the insertion of the probe members. This embodiment provides superior high-power-handling characteristics however due to the exposure of the conductive probe members within the waveguide section a slightly higher degree of phase shift variation will be noted over the previously described embodiment.

A very efiicient means for controlling high-power microwave energy has thus been disclosed. The control of the reflection coefficient, as well as absorption of the incident energy, is realized by varying the degree of coupling of novel and unique energy-absorbing means to a branch terminated transmission line. In terms of mechanical packaging in relation to prior art devices a considerably more compact integral unit will be realized in the practice of the invention. A feature of paramount interest will be the absence of frequency sensitivity which will enhance the utility of the invention in employment in all high-power microwave energy systems. The choice of the energy-absorbing materials is rather broad and may incorporate solid, as well as fluid, materials having the prerequisite dielectric properties depending on the level of the power to be handled and impedance-matching problems.

Numerous modifications, alterations and variations in structure, as well as the lossy energy-absorbing materials employed,

will readily occur to those skilled in the art without departing from the spirit and scope of the invention as defined in the appended claims. lt is intended, therefore, that the embodiments shown and described herein be considered as illustrative only and not in limiting sense.

What is claimed is:

l. A microwave energy control device comprising:

means for guiding electromagnetic energy along a first transmission line;

means for guiding said energy along a line oriented perpendicularly to said first line terminating in an energy reflecting member;

said terminated line including energy-splitting means and energy-absorbing means disposed transverse to the path of energy;

said absorbing means being removably inserted in said terminated line at a point spaced from said reflecting member;

and means for adjusting the depth of insertion of said energy-absorbing means to control the magnitude of power reflected back to a second transmission line.

2. A microwave energy control device according to claim 1 wherein said energy-absorbing means comprise a hollow element of a substantially lossy dielectric medium and means for circulation of a fluid coolant within said hollow element.

3. A microwave energy control device according to claim 1 wherein said energy-absorption means are coupled to said branch line by a conductive probe member.

4. A microwave energy control device comprising:

waveguide means providing a first transmission line; a line coupled perpendicularly to said first transmission line and terminating in a reflecting conductive wall member;

said terminated line including conjugate energy-splitting means and juxtapositioned waveguide means connected to said splitting means;

means for absorbing electromagnetic energy coupled to each of said terminated line waveguide means at a point spaced from said reflecting member and being adapted to be removably inserted within each terminated line waveguide mans;

and means for simultaneously adjusting the depth of insertion of said energy-absorbing means in each waveguide means to control the degree of coupling and the magnitude of reflected unabsorbed power returned to a second transmission line.

5. A microwave energy control device according to claim 4 wherein said conjugate energy-splitting means comprise a hybrid junction coupler.

6. A microwave energy control device according to claim 4 wherein said absorbing means include a hollow energy permeable member and a fluid coolant circulated within said hollow member.

7. A microwave energy control device according to claim 4 wherein said adjusting means comprise a common actuator cooperatively associated with each energy-absorbing means.

8. A microwave energy control device according to claim 7 wherein said common actuator includes a ganged conductive member permanently affixed to means supporting said absorbing means and threadably engaging a central elongated rotatable member.

9. A microwave energy control device comprising:

rectangular waveguide means having broad and narrow walls providing first and second transmission lines; a waveguide line coupled to a narrow wall of said first line and terminating in a reflecting conductive wall member;

said terminated line including a hybrid junction coupler having a common wall defining juxtapositioned passageways with a coupling iris opening and waveguide means connected to each of said passageways;

means for absorbing electromagnetic energy disposed transverse to the terminated waveguide means at a point spaced from said reflecting member and being adapted to be inserted a variable distance within said waveguide means;

an encircling conductive housing member secured to a broad wall of each terminated waveguide means;

and means for simultaneously moving said energy-absorbing means relative to said housing members to adjust the distance of insertion and control the degree of unabsorbed energy returned to said second transmission line.

10. in a microwave energy transmission system:

a source of high-power electromagnetic energy;

means for propagating said energy along a first transmission line;

means for guiding said energy along a line coupled perpendicularly to said first line and terminating in an energy reflecting member;

said terminated line including insertable energy-absorbing means shunting said terminated line at a point spaced from said reflecting member;

and means for adjusting the magnitude of insertion of the shunting means to thereby vary the propagation characteristics of the terminated line and the resultant power level of energy reflected back to a second transmission line.

11. A microwave energy transmission system according to claim 10 wherein said energy-absorbing means comprise a hollow element with a fluid coolant circulated therein.

12. A microwave energy transmission system according to claim 10 wherein said terminated line includes a hybrid junction coupler and separate waveguide means joined to said coupler with the energy-absorbing means supported by each said waveguide means.

13. A microwave energy transmission system according to claim 11 wherein said energy absorbing means are juxtapositioned and simultaneously adjusted by a common central control actuator. 

1. A microwave energy control device comprising: means for guiding electromagnetic energy along a first transmission line; means for guiding said energy along a line oriented perpendicularly to said first line terminating in an energy reflecting member; said terminated line including energy-splitting means and energy-absorbing means disposed transverse to the path of energy; said absorbing means being removably inserted in said terminated line at a point spaced from said reflecting member; and means for adjusting the depth of insertion of said energyabsorbing means to control the magnitude of power reflected back to a second transmission line.
 2. A microwave energy control device according to claim 1 wherein said energy-absorbing means comprise a hollow element of a substantially lossy dielectric medium and means for circulation of a fluid coolant within said hollow element.
 3. A microwave energy control device according to claim 1 wherein said energy-absorption means are coupled to said branch line by a conductive probe member.
 4. A microwave energy control device comprising: waveguide means providing a first transmission line; a line coupled perpendicularly to said first transmission line and terminating in a reflecting conductive wall member; said terminated line including conjugate energy-splitting means and juxtapositioned waveguide means connected to said splitting means; means for absorbing electromagnetic energy coupled to each of said terminated line wavEguide means at a point spaced from said reflecting member and being adapted to be removably inserted within each terminated line waveguide mans; and means for simultaneously adjusting the depth of insertion of said energy-absorbing means in each waveguide means to control the degree of coupling and the magnitude of reflected unabsorbed power returned to a second transmission line.
 5. A microwave energy control device according to claim 4 wherein said conjugate energy-splitting means comprise a hybrid junction coupler.
 6. A microwave energy control device according to claim 4 wherein said absorbing means include a hollow energy permeable member and a fluid coolant circulated within said hollow member.
 7. A microwave energy control device according to claim 4 wherein said adjusting means comprise a common actuator cooperatively associated with each energy-absorbing means.
 8. A microwave energy control device according to claim 7 wherein said common actuator includes a ganged conductive member permanently affixed to means supporting said absorbing means and threadably engaging a central elongated rotatable member.
 9. A microwave energy control device comprising: rectangular waveguide means having broad and narrow walls providing first and second transmission lines; a waveguide line coupled to a narrow wall of said first line and terminating in a reflecting conductive wall member; said terminated line including a hybrid junction coupler having a common wall defining juxtapositioned passageways with a coupling iris opening and waveguide means connected to each of said passageways; means for absorbing electromagnetic energy disposed transverse to the terminated waveguide means at a point spaced from said reflecting member and being adapted to be inserted a variable distance within said waveguide means; an encircling conductive housing member secured to a broad wall of each terminated waveguide means; and means for simultaneously moving said energy-absorbing means relative to said housing members to adjust the distance of insertion and control the degree of unabsorbed energy returned to said second transmission line.
 10. In a microwave energy transmission system: a source of high-power electromagnetic energy; means for propagating said energy along a first transmission line; means for guiding said energy along a line coupled perpendicularly to said first line and terminating in an energy reflecting member; said terminated line including insertable energy-absorbing means shunting said terminated line at a point spaced from said reflecting member; and means for adjusting the magnitude of insertion of the shunting means to thereby vary the propagation characteristics of the terminated line and the resultant power level of energy reflected back to a second transmission line.
 11. A microwave energy transmission system according to claim 10 wherein said energy-absorbing means comprise a hollow element with a fluid coolant circulated therein.
 12. A microwave energy transmission system according to claim 10 wherein said terminated line includes a hybrid junction coupler and separate waveguide means joined to said coupler with the energy-absorbing means supported by each said waveguide means.
 13. A microwave energy transmission system according to claim 11 wherein said energy absorbing means are juxtapositioned and simultaneously adjusted by a common central control actuator. 